We showed previously that Ca2+ and the Ca2+-binding protein, calmodulin, can act together to displace the low molecular mass GTPase, Rab 3A from macaque brain prefrontal cortex synaptic vesicle membranes. This finding was of potential interest because synaptic vesicle-bound Rab 3A is thought to influence the Ca2+-triggered release of neurotransmitters from axonal termini. The fact that Ca2+/calmodulin can displace Rab 3A from synaptic vesicle membranes in vitro raises the possibility that Ca2+/calmodulin may have a similar effect in vivo. During the past year we explored this possibility in a collaborative study with investigators at the University of Lausanne in Switzerland. The results of the study showed that a mutant form of Rab 3 that cannot bind Ca2+/calmodulin has no effect on stimulated exocytotic events. This suggests that the complex of Rab 3 with Ca2+/calmodulin that is normally generated in response to elevated concentrations of Ca2+ may reduce the numbe r of exocy totic events that occur in response to secretory stimuli. FUNDING NIH grant RR00166 and the Howard Hughes Medical Institute.